Modulation



July 2 1935. G. L. ussELMAN MODULATION Filed Sept. 16, 1932 GEORGE L.lELMAN A'TORNEY INVENTOR Patented July 2, 1935 MODULATION George LindleyUsselman, Port Jeerson, N. Y., assigner to Radio Corporation of America,a corporation of Delaware Application September 16, 1932, Serial No.633,431 zzfclaims. (ci. 17o- 171) This invention relates to signallingmeans and in particular to'I means whereby the characteristics of highfrequency oscillations other than amplitude are varied in accordancewith signals '-5 to be transmitted.

' It has been found that ordinary amplitude modulatedhighfrequency'oscillations in transmission from the sending station tothe receiving station are subject to what is known as fading effects. Inparticular, "the transmission of telegraph signals in the pastwasattendant by detrimental fading at thev receiver Where the diversityreceiving system was not in use. This is a decided disadvantage since itintroduces dropouts and errors in the signal. It lwas Alater found thatif the frequency band of the transmitted signal Were spread out or madewider the effect of this fading Was reduced.

` In the prior art the carrier frequency band was made Wider bymodulating the amplitude of the carrier energy at some desirable rate,usually in the neighborhood of 500 cycles per second. This produced sidebands at, say," 500 cycles above and-500 cycles below the carrierfrequency. The signal band was in this case 1000 cycles wide. Amplitudemodulation of a telegraph transmitter `makes it necessary to reduce thepower output' to a great extent.

It has been found that the carrier can be modulated in phase to spreadthefrequency band of the transmitted signal. This produces side bands oneach side of the carrier frequency, which also reduces the effects'offading at the receiver. Y

I have found that if the high frequency oscillations are modulated inphase or in frequency in accordance Withv the telegraph signal to betransmitted they are less subject to the effect of fading than amplitudemodulated Waves, assuming like amounts of power are utilized in thetransmission. Also the reason Whylphase or frequency modulatedoscillations in telegraph transmission are subject to a less extent tofading efffects than oscillations modulated otherwise is that a greateramount of the transmitter -power is available for transmission. In otherWords, a transmitter can operate at full power with phase or frequencymodulated telegraph signals, While it is necessary to reduce the poweroutput to labout one quarter-for amplitude modulation of the samesignals If We assume equal power output, reduction of fading should beslightly'greater Where the oscillations are modulated in phase orfrequency than when they are modulated otherwise.

l' When a high frequency carrier is modulatedA in phase or frequency inaccordance with telegraph signals, such modulation of the-,carrier isusually accomplishedfin a stage following the carrier frequencygenerator; which is usually a crystal control oscillator, or a longlinerrcrystal control oscillator, or any other kind of oscillationgenerator. Furthermore, the keying of the trans'- mitter in accordanceWith the telegraphy signals,

is usually accomplished in a stage subsequent to the modulator stage.Any standard method of keying may bev used.

'In each of my United States applications, Serial No. 623,558,'led July20, 1932, serial'No. 616,026, filed June 8, 1932, Serial'No. 602,487,filed April 1, 1932, and Serial No.v 607,932, filed April 28, i932, Ihave shown means for varying at signal frequency the characteristicsother than the amplitude of a carrier frequency. In each of thesearrangements the carrier frequency is impressed through phase shiftingVmeans on the control grids of a pair of 'thermi- Yonic tubes which havetheir anodes connected inV parallel-to a common tank circuit and theirinternal impedances varied in phase opposition by the signal wave. Ineach of these arrangements the phase modulator stagel comprises twotubes having their input electrodes symmetrically connected `andenergized as indicated above, and a common tank circuit connected inparallel to the anodes of said pair of tubes.l

In my United States applications Sen No. 633,955, filed September' 20,1932 and Ser..No. 637,923, filed October 15, 1932, I have shownimprovedtransmitters of the type referred to above. more thermionic tubes areused to accomplish phase modulation of the carrier frequency.

The present invention relates to an improved and simplified modulatingmeans by which the characteristics of a carrier Wave other thanamplitude maybe variedv at any desirable frequency to be transmitted bythe use of a single thermionic tube. 1

More in detail, this invention relates tov an improved modulating meansto reduce fading in a signal wherein the modulator stage comprises asingle tube having a control grid coupledjalternately to a source ofhigh frequency oscilla- An apparent advantage to be gained by the use ofthe present invention results fromthe In each of these transmitters oneor fact that the carrier frequency and modulating Waves are applied tothe thermionic modulator through a purely mechanical means of simple anddurable nature. This results in low rst cost and economic upkeep.

The novel features of my invention have been pointed out withparticularity in the claims appended hereto.

The nature of my invention and the operation thereof will be bestunderstood from the following detailed description thereof and therefromwhen read in connection with the drawing, throughout which likereference numerals indicate like parts, and in which:

Figure 1 illustrates a specific embodiment of the invention; ywhile,

Figures 2 and 3 illustrate a detail and a modication of the arrangementof Figure l.

The practice in prior art has been to use vacuum tubes to shift thephase in a phase modulator stage to phase modulate a telegraphtransmitter. To reduce fading in telegraph signals by phase modulationit is only necessary to modulate the phase of the carrier frequency 4ofthe transmitter at some desirable but uniform frequency, usually between300 and 1000 cycles per second. The'present invention does this simplywith a phase splitting circuit and a motor driven rotary condenser. Theway in which' this is` accomplished is described below in connectionwith the drawing.

A specific embodiment of my invention is shown schematically in Figuresl, 2 and 3. Referring to Figure l, the motor A drives the shaft S ofrotary condenser C-XC through the insulated shaft coupling Y. The upperpart C of the rotary condenser is connected to the grid G of tube Vthrough the phase shifting inductance coil D while the lower part of therotary condenser is connected to grid G of tube V through the phaseshifting capacity or condenser E. Tube V is shown as the screen grid orfour element type, but by using a neutralized circuit a three elementtube may be used.

vDirect current biasing potential for the grid G of tube V is suppliedby way of choking inductance Q (a resistor may be used if preferred)from a potentiometer 5 connected as shown in parallel with a portion ofsource I 0. A low impedance path for alternating currents passingthrough Q is provided by condenser 6 connected as shown between Q andthe grounded lead of the heating circuit for cathode F. Alternatingcurrent in tank circuit YM is shunted around source l0 by means of aby-pass condenser 8 connected as shown. Charging poten- ,tial for thescreen electrode I2 of tube V is provided by a lter circuit comprising alead 8 tapped to a point on potentiometer resistance I4 and a condenserconnected as shown.

The output of tube V and tank circuit M is connected by a line R tostage N, which may be a limiter, a frequency multiplier, an amplier, orall three combined. Stage N is coupled to stage O by lines S-S. Stage Omay be a frequency multiplier, an ampliiier, or both. The output ofstage O is connected to a load circuit, which is the transmission lineand antenna P in this case. The oscillator B is connected totherotatable element X of the rotary condenser by the sliding contact Zwhich rests on the element which rotates the plates X.

Figure 2 shows an end view of the rotary condenser used in the modulatorof Figure l. As shown here, the `movable plates X, which are connectedwith the high frequency oscillator, are driven by an element S relativeto the xed plates C' and C. The plates C and C are conductivelyconnected as shown to the phase shifting elements D and E respectively.

The transmitter keying means has not been shown but it may be keyed ormodulated by any of the well known methods. The modulating or keying ofthe energy may be accomplished in any stage following the phasemodulator, as indicated in the drawing.

The operation of the phase modulator will now be set forth. In thefollowing statement cf the operation assume that the motor A is drivingthe shaft S and the rotating element X of the rotary condenser X-C-C ata speed to give the desired rate or frequency of phase shift ormodulation. We may assume this speed gives, say, 50) cycles per secondmodulation. If the oscillator B is delivering constant high frequencyenergy, say 2,000,000 cycles per second to X by way of contact Z and theshaft S of the rotary condenser, the two stationary halves C' and C willbecome excited with high frequency energy alternatingly at a rate of 500cycles per second (assumed). The insulator Y prevents this excitationfrom B from escaping to ground through the motor A. The excitation fromC passes to the grid G through the phase retarding inductance D. Theexcitation from C passes to the grid G through the phase advancingcapacity E. Since C and C are alternately excited at a 500 cycle rate,the phase of the high frequency excitation reaching the grid G will beretarded and advanced in phase at a 500 cycle rate. Consequently thephase of the carrier frequency transmitted or radiated by the antennawill be phase modulated at a 500 cycle rate or at a rate determined bythe speed of the rotary condenser C-X-C. Amplitude limiters or frequencymultipliers may be used in the transmitter before the signal is finallyamplified and radiated by the antenna. Consequently the signalcharacters radiated from the transmitter will be modulated in phase atthe assumed 500 cycle rate.

The operation of the rotary condenser is simply to capacitively couplethe oscillator B alternately to the excitation phase retarding element Dand the excitation phase advancing element E, from which elements theexcitation energy is conducted to the grid G of tube V. The anodecurrent of tube V causes the oscillations in tank circuit M to be phasemodulated and from this circuit the radio frequency in the followingstages of the transmitter is phase modulated at a regular rate. Thecarrier energy of the transmitter may be keyed or modulated by thesignal with any suitable means before it is radiated by the antenna.This keying or interrupting of the phase modulated carrier energy may beaccomplished in any stage of the transmitter. Preferably, keying,modulating, or interrupting of the phase modulated carrier isaccomplished in any stage subsequent to the modulator stage. Forexample, this operation may be accomplished as indicated either in stageN or O.

When it is desired to obtain a higher modulating frequency withoutincreasing the speed of the motor A an arrangement as shown in Figure 3may be used. In this compound rotary condenser of Figure 3 the rotatableelement X takes the form of a disk with teeth fastened to the pe ripheryof the rotating disk which is mounted on a shaft S for rotation bythemotor A. The plates C, C are "mounted adjacent the pathof rotation ofthe teeth of X so that the capacity between the disk X and the platesC', C alternately increase and decrease. Thisalternate increase anddecrease ofV coupling is Ytransferred by way of phase shifting means Dand C to the control electrode G of tube V, the same as 'in Figure l.Due to the fact that.v there area plurali'ty of teeth X on the disk 14,a lsingle rotation of the shaft S alternately Vincreases and decreases.the coupling lbetween the lrotatable disk I4 and the fixed plates C', Canumber of times. The capacity X-C-C' maycomprise a single disk X andsingle platesC', Cor a plurality of disks X and a pair of xed plates C',C located adjacent the periphery of each of the disks X so that thecoupling between the disk X and the plates C', C will alternatelyincrease and decrease as the teeth` of the disk X move toward theiates-CQ C.

Having thus described my invention and the operation thereof, what I`claim is:

` 1. Phase modulating means comprising la source of high frequencyoscillations, a-fthermionic tube, a rotatable condenser, a connectionbetween one armature-of said condenser and the control electrode of saidtube, a connection `between the other armature of said condenser andsaid source of high frequency oscillations, and a motor for rotatingsaid condenser at a modulating frequency rate.

2. Phase modulating means comprising a sourceof high frequencyoscillations, a thermionic tube, parallel variable capacities connectedin series between the control, electrode of said tube and said highfrequency oscillation source, and means for varying said capacities inopposite sense at a modulating frequency rate.

' 3. Modulating means comprising a source of high frequencyoscillations, a thermionic tube, parallel variable condensers connectedin series between the control electrode of said tube and said highfrequency oscillation source, and means for alternatelyV increasing anddecreasing the capacity of said condensers atv a modulating n frequencynote.

control electrode of said tube, and mechanical means for coupling saidconductors to said' source of oscillations and for changing ythe valueof the coupling between the respective conductors and said sourcebetweenr predetermined maximum and minimum values in a complementarysense. v

6. Modulating means comprising a thermionic tube, a work circuitconnected between ythe output electrodes of said tube, a pairl of pathsof different electrical characteristics connected to the controlelectrode of said tube, the other terminals of said paths'beingconnected tothe statorplates oi a condenser, a source of high fre- "of,said condenser, andfmeans for rotating the vrotatable plates of saidcondenser at a rateof the order of 1an audible frequency.

7. Signal modulating means comprising -a source oi high frequencyoscillations, a thermif' .onicrelay tube, Va tuned circuit connected,be-

tween the output electrodes of said tube, a pair of paths, of different`electrical characteristics connected to the control electrode of saidtube,

`the otherterminals of said paths being connected :to the stator platesof a condenser havingtwo stators and a rotor, means for connecting saidsource of high frequency oscillations to the r0- tor plates of saidcondenser, and means for rotating the rotatablefplates of saidcondenser.. Y;

8. Signal modulating means comprising ra source of high frequencyoscillations of constant frequency, a thermionic tube, a tuned tankcircuit connected between Vthe output electrodes of said tube; a pair ofpaths connected-to the control electrode -of said tube, one of saidpaths including a phase retarding element, the other of said pathsincluding a phase .advancing element, means for connecting the otherterminal of each of said paths ,to a stator of ay condenser having twostators and ya rotor, means for connecting said source of high frequencyoscillations to .the rotor of said condenser, and means for rotating therotor. of said condenser at a desired modulating frequency rate.

9. Signal modulating means comprising a thermionic tube having input andoutput electrodes, a work circuit connected to the output electrodes ofsaid tube, a pair of transmission channels of different electricalcharacteristics connected to Ythe input electrode of said thermionictube, a

compound condenser comprising a pair of fixed plates'anda set ofrotatable plates, aconnection between each of said transmission channelsand one of said iixed plates, a source of high frequency oscillationsconnected to said rotatable plates, and means forY rotating saidrotatable plates at a desiredfrequency rate.

10. The combination of a frequency multiplier, a thermionic relay tubehaving its output electrodes connected thereto, a compound condensercomprising a .movable plate, a pair of fixed plates, reactances ofdifferent character' lconnecting said fixed plates to the input elec-vtrodes of said thermionic relay tube, said movable plate being .adaptedto be rotated at a rate equal'to an audible frequency, and a highfrequency oscillator connected to said movable plate. f.

11. Phase modulating means comprising in combination a thermionic tube,a variable capac- Y ity having a pair of Xed elements and a rotatableelement, said fixed elements each being connected through seriesreactances to the input electrodes of said tube, a source of highfrequency oscillationsconnected to vsaid rotatable element, `and means.for driving said rotatable element. i

, l2. The combination of afrequency multiplier, an amplitude limiterconnected thereto, a thermionic tube having its output electrodesconnected to said limitera 4condenser comprising a pair of fixed plates:connectedto the input electrodes of saidthermionic relaytube saidVcondenser also comprising a rotatable plate of mechanical means forVrotating said ,rotatable plate, and a high'frequency oscillatorconnected to said rotatable plate.

13. Signal modulating'. :means comprising a quency oscillationsconnected to the rotor plates: -themoll tube having input and Output2160-775 trodes, a tank circuit connected to the output Ielectrodes ofsaid tube, a pair of `transmission channels connected to the inputelectrode of ksaid `thermionic tube, one of said channels beinglcalgzacitive inv character, the other of said chan- -nels beinginductive in character, acondenser comprising a pair Vof fixed platesand a movable plate, a connection between each of said transmissionchannels and one of said fixed plates, a source of high frequencyoscillations connected to said rotatable plate, means for vrotating saidrotatable yplate at a low frequency rate, means -for .tuning said tankcircuit to a frequency which is a multiple of the frequency of saidsource, and means for keying or modulating the resulting phase modulatedhigh frequency energy at signal frequency.

14. Signal .modulating means comprising a thermionic tube having inputand output yelectrodes, a tank circuit connected to the outputelectrodes of said tube, a pair of transmission channels `connected tothe input electrode of said thermionic tube, one of said channels beingcapacitive in character, the other of said chanfrequency, and radiatingmeans coupled to said .tank circuit.

15. Wireless telegraphy means comprising a source of high lfrequencyoscillations, means for wobbling the phase of said oscillationscomprising a. vthermionic tube, said tube having input .and outputelectrodes, capacities connecting the input electrode of said tube tosaid source of high frequency oscillations by way of reactances ofunlike character, means for varying said capacities at a rate equivalentto an audible frequency and keying means connected with the output ofsaid tube.

16. Telegraphy signalling means comprising, a thermionic tube havinginput and output electrodes, a pair of conductors of diierentelectricalcharacteristics connected to the input electrode of said tube,means for applying high frequency oscillations to said conductors, saidoscillations being alternately applied at a rate equivalent to anaudible frequency, a work circuit coupled to the output electrodes ofsaid tube, and keying means interposed between said vwork circuit andthe output electrodes of said tube.

17. Telegraphy means comprising a thermionic tube having rinput andoutput electrodes, a pair of conductors of different electricalcharacteristics connected to the input electrode of said tube, means foralternately applying high frequency oscillations to said conductors,said oscillations being alternately applied at low frequency rate, atank circuit connected between Vthe output electrodes of said tube, saidtank circuit being tuned to a multiple of the frequency ofthe appliedhigh frequency oscillations, a utilization circuit, and relaying meansincludin keying means connecting said tank circuit to said utilizationcircuit.

, .18.111 a telegraph .transmitter 'the .combina- Vacca-co9 tion of,phase modulating means, frequency multiplying means and amplifying meanscoupled thereto, said phase modulating means comprising Aa thermionictube having input and output electrodes, a pair of conductors ofdifferent electrical characteristics connected to the input electrode,means for alternately applying high frequency oscillations to saidconductors, said oscillations being applied at a low frequency rate, andmeans coupled to the output electrodes of said tube for keying ormodulating the phase modulated carrier energy in accordance with thesignals to be transmitted.

19. Signal modulating means comprising, a thermionic tube having anoutput circuit con nected between its output electrodes and a directcurrent circuit including an impedance and a source of potentialconnected between its input electrodes, a condenser comprising a pair ofxed plates and a stator plate mounted adjacent said xed plates, a motorand a source of high frequency oscillations connected to said statorplate, a capacity having one terminal vconnected to the control grid ofsaid tube and an inductance connecting the other terminal of saidcapacity to one of said xed plates, and a capacity connecting said otherterminal of said condenser to the other fixed plate of said condenser.

20. Signal modulating means comprising, a thermionic tube having aninput electrode, a pair of high frequency conductors of differentelectrical characteristics connected to the input electrode of saidthermionic tube, a compound condenser comprising a pair of xed platesand a movable plate, a connection between each of said transmissionlines and'one of said fixed plates, a source of high frequencyoscillations connected to said movable plate, and means for moving saidmovable plate relative to said fixed plates at a modulating frequencyrate.

21. A transmitting system comprising a source of high-frequencyoscillations, an inductive circuit for said oscillations, a capacitivecircuit for said oscillations, means to continuously (cause,alternately, maximum energy to flow from said source through saidcircuits, means combining the energy fiowing through said circuits, saidcombined energy being in the form of oscillations continuously varied inphase, means to impress a signal to be transmitted upon saidphase-varied oscillations, and means for transmitting said oscillationsbearing said phase variations and said signal.

22. In a transmitter for reducing fading effects, a source ofhigh-frequency oscillations, aninductive circuit, a capacitive circuit,each of said circuits being connected to said source, means forincreasing the iiow of oscillations from said source through saidinductive circuit while decreasing the flow of oscillations from saidsource through said capacitive circuit, and for decreasing the flow ofoscillations from said source through said inductive circuit whileincreasing the flow of oscillations from said source through saidcapacitive circuit, means for combining the oscillations ilowing throughsaid circuits whereby said combined oscillations form oscillationscontinuously varying in phase, means for amplifying said oscillationsvarying in phase, means for impressing a signal on the amplifiedoscillations, and means for transmitting said oscillations bearing saidsignal.

`GEORGE LHTDLEY USSELMAN.

